The present invention relates to mono-specific and multi-specific polypeptides that specifically bind or interact with CD3. These polypeptides can be, but are not limited to, antibodies, fragments thereof, scFvs, Fabs, di-scFvs single domain antibodies, diabodies, dual variable domain binding proteins and polypeptides containing an antibody or antibody fragments. In one embodiment, a multi-specific polypeptide binds both a T-cell receptor complex on T-cells and a tumor antigen to induce target-dependent T-cell cytotoxicity, activation and proliferation.